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Cold Room / Cold Frame: Key word here is “cold.” Generally used to over-winter your deciduous trees. Typically our winters are not severe enough to require this type of protection, but there are some definite benefits. First, even though our winter is not real cold [ed.: Indiana, in case you missed it above], some species can suffer damage. Most damage is usually apparent in the branching.Fine branches can be broken in the wind or some may have slight ‘die back.’ Once you have a tree with nice ramification, you don’t want to lose all that hard work. So, these structures are first used as shelters. The down, or maybe good, side to these structures is heat created by the sun. Sun and warmth can fool a plant into thinking it’s spring, so your trees generally will awaken early. Temperatures inside these structures should be closely monitored. Do it manually by opening a door or lifting the lid of a cold frame on sunny days, or automatically via motorized vents and fans. Generally it is recommended to go automatic here. I have seen temperatures of over 75 degrees during February on sunny days. All it takes is one or two days of sun during which you forget to open the shelter. Also on these sunny days, if your glass area is close to the tree tops, you can experience death of the trees’ upper areas. Remember, glass can intensify the heat of the sun.

Next, no matter how you vent it, your trees will eventually awaken earlier that their brethren outside. This can be good, or bad. Remember, once awakened, they need to continue receiving good light and temperatures. A drop in either can cause unwanted growth – leggy due to low light, or damaged foliage from cold temperatures. The good side, if you maintain proper temp levels once they’re awakened, you can start your spring work earlier and actually extend your trees’ growth season.

Greenhouse: Greenhouses are for starting new plants or growing plants on through the winter. Generally in Bonsai, you don’t need a greenhouse except for tropicals. There are, though, a couple of ways to use your greenhouse. You can simply maintain your tropicals through winter in a warm atmosphere by maintaining a minimum temperature of around 50-60 degrees.Tropicals will exist but not grow in these lower temps. If you increase your temperatures and maintain a minimum of 70 degrees, they will actually grow through the winter. If you want better growth, you must add supplemental lighting. Utilizing HPS (High Pressure Sodium) lights and a minimum of 70 degrees, your tropicals will grow very well – maybe not quite as well as in summer outside, but they will do well. The other huge benefit of a greenhouse is humidity. Tropicals love humidity.

Now, let’s take the next step. I am in the lucky position to have both a cold room and a heated greenhouse — the best of both worlds. Generally, my deciduous trees stay out until Christmas. They then go into my cold room (an actual room within my greenhouse). Luckily, my cold room maintains a minimum temperature above freezing. Actually it probably averages around 40–50 degrees. The reason my trees stay out till Christmas is to make sure they enter dormancy and stay dormant prior to coming in to the cold room. By the end of January I am ready to start working on my deciduous trees. I bring them into the greenhouse, prune, wire, and repot them if needed. They then go back to the cold room. By mid-February I can maintain a temperature of above 50. My trees start to bud by late February. In March, I now average a temperature of around 60 degrees. My trees start their spring probably 2 months early. The downside to all this is my trees must stay in until late April so they don’t get shocked by a late cold spell outside.

The huge benefit of greenhouses is starting early. There’s nothing better during a cold winter day than working on your trees.

Winter Heat & Light

For those of you who have greenhouses with grow-lighting systems, consider this: Generally in winter the coldest period at night is just before sunrise – furnace working its hardest at this point. If you have your lights set to come on in the a.m., consider turning them on earlier in the morning and maybe not run them as late at night. My lights are set to come on at 4:00 a.m. and turn off at 8:00 p.m. thus 16 hours of strong light for growth. Almost all types of grow-lighting systems add some heat in your greenhouse. You might as well utilize it during the cold part of the morning. If you question my timing, consider how long our day is during the highest growth periods of summer. I maintain a growing atmosphere in my greenhouse.